The present invention relates to rubber-reinforced monovinylidene aromatic polymer compositions of the type commonly referred to as xe2x80x9chigh impact polystyrenexe2x80x9d or xe2x80x9cHIPSxe2x80x9d. More particularly, the invention relates to such compositions in which the rubber-reinforcing particles are produced from a specific polybutadiene rubber, have a specific particle size distribution and a process for the production of such compositions.
Rubber-reinforced polymer compositions of the HIPS type are widely used in many applications because of their ease of molding, good gloss, and generally good mechanical properties. It has been known for some time that improved combinations of gloss and mechanical properties can be achieved in such rubber-reinforced polymers by providing a so-called xe2x80x9cbimodalxe2x80x9d distribution in the sizes of the rubber reinforcing particles, i.e. the particles show two distinct peaks in their size distribution. This can be achieved by combining two or more such resins or components, each having a group of rubber particles having a different average particle size. Various monovinylidene aromatic polymer compositions are known having at least two groups of rubber particles wherein the groups have different average particle sizes. See for example U.S. Pat. Nos. 4,146,589; 4,214,056 and 4,334,039 and European Patents 0 096 447, 0 158 258 and 0 152 752 which disclose such compositions.
U.S. Pat. No. 4,493,922 also discloses rubber-reinforced polystyrene compositions having bimodal rubber particle size distributions. The average rubber particle sizes disclosed for the groups of particles are 2 to 8 micrometers (especially from 5 to 6 micrometers) for the group of larger particles and 0.2 to 0.6 micrometers for the group of smaller particles.
As mentioned, a number of methods are proposed for achieving such a bimodal particle distribution. For example, U.S. Pat. No. 4,153,645 discloses a method for the preparation of a HIPS-type polymer in which two polymer compositions are prepared using standard production processes, the compositions having different average particle sizes. These two polymer compositions are then mixed by a subsequent mechanical blending process.
An alternative approach to producing HIPS polymers with a bimodal rubber distribution has been to introduce feed streams of monomer and rubber at two different points in the polymerization system. This results in a polymer product which generally has a fairly broad spread of rubber particle sizes. Examples of this are described in U.S. Pat. No. 4,334,039 and EP 0 096 447. A disadvantage of such methods is that the mechanical properties of the resulting product can be somewhat poor and difficult to control.
Yet a further approach is disclosed in U.S. Pat. No. 4,146,589 and EP 0 048 389. In this method, two prepolymer compositions are prepared containing rubber particles with different particle sizes. The prepolymer compositions are then mixed and further polymerized to provide a polymer having a bimodal particle size distribution.
Other references in this area include EP-418,042 wherein the rubber comprises a partially coupled radial or star rubber, having a cis content of less than or equal to about 70 percent, JP 02762722 wherein the rubber is a mixture of a high cis polybutadiene of high molecular weight and a low cis polybutadiene of low molecular weight, and JP 95005789 wherein the rubber is a mixture of a high molecular weight polybutadiene and a low molecular weight polybutadiene, both having a cis structure of greater than 80%. However, a desirable balance of impact strength and tensile modulus is still not attained for some specific applications.
Therefore, it is still desirable to obtain a rubber modified monovinylidene aromatic polymer having an improved balance of impact and tensile properties for select applications.
The present invention is a rubber modified monovinylidene aromatic polymer comprising:
a) a monovinylidene aromatic polymer matrix,
b) rubber particles dispersed therein, characterized in that the rubber particles are produced from a diene rubber having I) a high molecular weight component and II) a low molecular weight component; the high molecular weight component having a weight average molecular weight at least two and one half times greater than the weight average molecular weight of the low molecular weight component, wherein both components I and II have a 1,4 cis content of greater than 70 percent and III) the rubber is grafted with monovinylidene aromatic polymer to the extent that there is at least 30 percent monovinylidene aromatic polymer present as grafts on the rubber.
In a preferred embodiment the present invention is a rubber modified monovinylidene aromatic polymer comprising:
a) a monovinylidene aromatic polymer matrix,
b) rubber particles dispersed therein in the form of small and large particles, wherein the volume average particle diameter of the small particles is from about 0.1 to about 2 micrometers and the volume average particle diameter of the large particles is from about 2 to about 6 micrometers, characterized in that the rubber particles are produced from a diene rubber having I) a high molecular weight component and II) a low molecular weight component; the high molecular weight component having a weight average molecular weight at least two and one half times greater than the weight average molecular weight of the low molecular weight component, wherein both components I and II have a 1,4 cis content of greater than 70 percent and III) the rubber is grafted with monovinylidene aromatic polymer to the extent that there is at least 30 percent monovinylidene aromatic polymer present as grafts on the rubber.
Another aspect of the present invention is a process for preparing a rubber-modified monovinylidene aromatic polymer comprising the steps of: (a) continuously supplying a reaction mixture comprising monovinylidene aromatic monomer and a dissolved diene rubber to a reactor means, (b) continuously polymerizing the monovinylidene aromatic monomer in the presence of the dissolved diene rubber in the reactor means under conditions whereby phase inversion subsequently occurs, (c) continuously removing from the reactor means a diene rubber-reinforced monovinylidene aromatic polymer, which process is characterized in that: (d) the diene rubber which is dissolved in the reaction mixture has distinct high and low molecular weight components, the high molecular weight component having a weight average molecular weight at least two and one half times greater than the weight average molecular weight of the low molecular weight component, both components having a 1,4 cis content of greater than 70 percent.
In a preferred embodiment, the process further comprises (e) wherein the process conditions prior to phase inversion are adjusted to produce different groups of diene rubber particles from the high and low molecular weight components of the diene rubber, each group having a different average rubber particle size.